[Mesa-dev] [PATCH 22/22] meta/blit: Use GL_EXT_shader_samples_identical in MSAA-SS resolve blit

[Neil Roberts]

Iago Toral <itoral at igalia.com> writes:

> I don't know much about this, but using shader_samples_identical
> should only give a benefit if we actually get identical samples,
> otherwise it means more work, right? I noticed that the test renders a
> quad with random colors for each vertex that will be interpolated
> across the region, so could it be that we are hitting few cases of
> identical samples? Shouldn't the results be expected to be better if
> we rendered a flat color instead?

It should all be identical regardless of the color chosen because it is
doing per-pixel shading rather than per-sample which means the fragment
shader is only run once per pixel and the same value is written to all
of the samples. The only case where you get differing samples normally
is at the edges of a polygon where the single color is only written to a
few of the samples.

In this case it is even more identical because the random color is set
on a uniform rather than a vertex so it is actually using the same color
for the whole framebuffer. However, it is rendered as two triangles
which might mean that along the diagonal of the framebuffer where the
two triangles meet there will be a polygon edge which means the hardware
might not recognise that these all the same color, but that should only
be a few pixels and probably won't affect the results very much.

Regards,
- Neil